Abstract

Soluble proteins are poorly immunogenic, and will often lead to Ag-specific tolerance. Similarly, Ag associated with apoptotic cells will frequently induce tolerance. These types of Ag are not very “dangerous”, so that recruitment and stimulation of professional APC is minimal. Treatment with the hematopoietic growth factor Fms-like tyrosine kinase 3 ligand (Flt3L) results in tremendous DC expansion, and previous studies have shown that Flt3L treatment can prevent the establishment of tolerance. Yet, the exact mechanism by which Flt3L treatment enhances immune responses remains undefined. I.V. injection of apoptotic cells or soluble Ag into wt mice induces tolerance, but this tolerance is abrogated by treatment with Flt3L. Interestingly, similar abrogation of tolerance was not observed in CD154-/- mice, suggesting a role for CD40-CD40L interaction. Further analysis of Flt3L-treated mice found reduced expansion of CD11c+ cells in CD154-/- mice versus B6 mice. This reduction in CD11c+ cells correlated with reduced T proliferation, as determined by CFSE dilution, in Flt3L-treated CD154-/- mice compared to B6 mice. Phenotypic analysis of the CD11c+ DC from Flt3L-treated B6 and CD154-/- mice showed reduced costimulatory molecule expression, especially CD40, in the CD154-/- CD11c+ DC. Together, these data suggest that maximal DC mobilization by Flt3L requires CD40-CD154 binding, which primes the DC for stimulating immunity even against Ag that normally induce tolerance.